Microwaveable adhesive article and method of use

ABSTRACT

An adhesive article for use in a microwave oven is provided having a hot-melt or heat-curable adhesive substrate, and a susceptor layer of electrically conductive or semi-conductive microwave absorbing material that is disposed on at least a portion of the substrate. The article becomes less absorbent of microwave radiation upon the melting or deformation of the substrate. This article is suitable for adhering one adherend to another by exposing said article containing a hot-melt adhesive substrate to microwave energy.

This is a continuation-in-part of Ser. No. 725,188 filed Jul. 3, 1991,now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to microwaveable adhesives. More particularly,this invention relates to hot melt adhesives or heat-curable adhesivesthat are activated by microwave energy.

2. Background of the Invention

Microwave radiation has become a widely used means for heating objects,particularly food. One advantage of microwave heating is that objectsmay be heated more quickly relative to conventional heating methods.

Microwave heating involves the portion of the electromagnetic spectrumbetween infrared and radio waves. Microwaves heat materials having anelectric dipole. Water is by far the most common dipolar material.Typically, microwaves pass through an object and tend to align thedipoles. The microwave field reverses itself billions of times a second,which tends to cause the dipoles to rotate. The energy which is releasedby the rotating dipoles is converted into heat.

Polar materials such as water are not the only microwave receptors.Electrically conductive materials such as metals are also microwavereceptors. However, the metal must be very thin or it will reflectalmost all of the incident microwave energy rather than absorb themicrowave energy.

An electrical conductor of proper thickness undergoes joule heating whenirradiated with microwave energy. If a conductive layer is disposed on anonconductive substrate, the substrate is heated by the transfer ofthermal energy from the conductive layer. Joule heating of a conductivelayer is generally much more efficient than simple dipole heating andresults in heating rates that can be orders of magnitude greater thanthe heating rates accomplished through the interaction of microwaveenergy with dipoles.

When microwave energy impinges upon a conductive layer, the microwaveenergy induces electronic motions that give rise to a current in theconductive layer. Since the conductive layer has a resistance, energy inthe form of heat, H, will be dissipated in accord with Joule's law whichis

    H=I.sup.2 R

where I is the current in the conductive layer in amperes and R is theresistance of the layer in ohms. Joule heating, however, takes placeonly as long as the conductive layer remains electrically continuous. Ifthe conductive layer becomes electrically discontinuous, the current isreduced or eliminated and joule heating is correspondingly reduced oreliminated.

In addition, microwave radiation generated in a microwave oven is notalways uniformly distributed throughout the oven. This non-uniformitycan give rise to differential heating of the various regions in anobject to be heated. Where the amount of microwave radiation is higher,the object heats more rapidly in that region and a hot-spot results.

Microwave radiation can also be used to heat hot-melt and heat-curableadhesives to their melt-flow or activation temperatures. The use ofhot-melt adhesives in industry has steadily been increasing in pastyears, replacing aqueous and solvent-based adhesives. Hot-melt adhesiveare particularly preferred because they do not release solvent into theatmosphere and also enjoy rapid set time characteristics. A hot-meltadhesive must be heated to its melt-flow temperature in order to allowthe adhesive to flow and bond onto the surface of the adherends. Thisheating time is generally very short, but may be quite long particularlyif the adhesive is in contact with a large mass that can act as a heatsink, or when the adhesive is separated from the heat source byinsulating materials. In such circumstances, there exists a need for amethod to heat up hot-melt adhesives at a rate faster than conventionalthermal heating.

Heat-curable adhesives are adhesives that are chemically activated uponexposure to heat. The adhesives form bonds either when water or solventis driven off, or when they are cross-linked, crystallized or otherwiseinitiated after exposure to heat.

U.S. Pat. No. 4,906,497 discloses microwave-activatable hot-meltadhesives. This reference describes an adhesive with an electricallyconductive substance blended into the adhesive. The electricallyconductive substance heats up faster than the adhesive, transferring theheat to the adhesive.

SUMMARY OF THE INVENTION

The instant invention provides a microwave actuable self-limitingadhesive article for use in a microwave oven which can be used to bondadherends. This adhesive article comprises:

a. a substrate consisting of a hot-melt or heat-curable adhesive; and

b. a microwave susceptor layer of at least electrically semi-conductivemicrowave radiation absorbing material. The susceptor layer is disposedon at least a portion of the substrate, and is responsive to exposure tomicrowave radiation for raising the temperature of the substrate above adesired level sufficient to melt the substrate. The susceptor layer isdeactivated when this temperature level is achieved.

This invention additionally provides for buff pads and a method ofmaking them using the article wherein said substrate is a hot-melt orheat-curable adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the disclosed article.

FIG. 2 is an exploded view of a buff comprising the microwave heatableadhesive substrate of the present invention.

FIG. 3 is an exploded view of the structure used to assemble the buff ofFIG. 2.

FIG. 4 shows graphs of the time versus the temperature of hot-meltadhesive at the edge and at the center of a buff assembly as it isexposed to microwave radiation.

FIG. 5 is a graphical representation of the time to heat an the hot-meltadhesive of the present invention at various vapor coat thicknesses.

FIG. 6 is a close-up view of a section of the graph shown in FIG. 5.

FIG. 7 shows the heating curves of various compositions of hot-meltadhesive when subjected to microwave radiation.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The instant invention comprises a heat meltable or heat deformablesubstrate and a microwave susceptor layer of at least semi-conductivemicrowave radiation absorbing material that is disposed on at least aportion of the substrate.

By "self-limiting", it is meant the rate of heating of the adhesivearticle is reduced once the appropriate temperature level is reached, sothat the adherends do not experience excessive temperatures.

FIG. 1 shows a cross-section of the disclosed article 10. Article 10comprises substrate 12 and a microwave absorbing thin layer 14.

The adhesive substrate may be any material that will melt or deform whenheated to a predetermined temperature in a microwave oven. The adhesivesubstrate may be selected such that the melt temperature isappropriately matched with the requirements of the adherends. Forexample, a low-melting point adhesive is appropriately used foradherends that are sensitive to high temperatures. Very high meltingpoint adhesives may be used where the ultimate product will be exposedto high temperatures in normal use. Preferably, the substrate has lowtoxicity, especially when melted or deformed, has no strong orunpleasant odor. There is essentially no limitation on the types ofhot-melt adhesives useful in the present invention. Hot-melt adhesivesare adhesives that are applied to adherends in a molten stage, and forma bond on cooling to the solid state of the adhesive. Common hot-meltssuch as ethylene vinyl acetate, ethylene ethyl acrylate, ethyleneacrylic acid, ethylene methacrylic acid, polyamides, polyethyleneethylene vinyl esters and mixtures thereof are just a few of the manytypes of hot-melt adhesive substrates envisioned for the presentarticle. Additional examples include the alkyds, coumarone-indene,phenolic, rosin or terpene resins and mixtures thereof. A preferredhot-melt adhesive is a ethylene acrylic acid/polyethylene blend.

Heat-curable adhesives are activated on exposure to heat. For example,heat-curable adhesives may be activated by driving off water or solventthrough the heating process. Alternatively, a heat-curable adhesive maybe cross-linked or crystallized on exposure to heat. Examples of suchadhesives include phenolic and amino resins, nitrile and neoprenerubbers, epoxy resins, certain isocyanate polymers and certain vinylmonomers such as methyl methacrylate and methyl 2-cyanoacrylate.Mixtures of these heat-curable adhesives are also contemplated.

Hot-melt or heat-curable adhesives are typically commercially availablein sheet form in predetermined thicknesses. Alternatively, the adhesivesmay be obtained in a net or mesh format.

Additives may be added to the hot-melt or heat-curable adhesives such astackifiers, fillers, antioxidants, waxes and other additives common tohot-melt or heat-curable adhesives.

The microwave susceptor layer is formed from a layer of metallicelectrically conductive or semiconductive material. The susceptor layerof conductive material can be made of a single metal, a mixture ofmetals, an oxide of a metal, a mixture of oxides, or any combination ofthe foregoing. Preferably, the microwave susceptor layer has a sheetresistivity of greater than 0.2 ohm per square. Sheet resistivity isdetermined according to ASTM D 257-78 (reapproved 1983). The susceptorlayer is generally less than 1% of the weight of the article and isoften less than 0.01%. Metals that are suitable for the susceptor layerinclude magnetic metals; such as iron, nickel, magnetic stainless steel,or alloys; and non-magnetic metals such as aluminum, tin, tungsten,non-magnetic stainless steel, titanium, silver, gold, magnesium, copper,chromium or alloys.

The microwave susceptor layer can be applied to the substrate by meansof metallizing processes such as evaporative vacuum deposition, sputterdeposition, electroplating, electroless plating or other appropriatemetallization processes.

The microwave susceptor layer preferably is sufficiently thin to preventreflection, but it must also be sufficiently thick to absorb sufficientmicrowave energy for its intended purpose. The thickness of themicrowave absorbing susceptor layer can preferably vary from 60 to 1,000angstroms, and more preferably 70 to 170 angstroms for deposited metalsand 200 to 2,000 angstroms for metal/metal oxide deposits. It ispreferred that the resistivity of the microwave absorbing susceptorlayer be uniform over its surface. A thicker layer may be required wherethe surface of the substrate is rough in order to assure the presence ofa continuous microwave susceptor layer.

Another embodiment of the invention provides a coating of electricallyconductive polymer that acts as a microwave radiation absorbingsusceptor layer. Examples of such polymeric coatings includepolypyrrole, polyaniline and polythiophene polymeric coatings whereinthe polymers have been so doped to form electrically conductivecoatings. To form these coatings, the monomers are polymerized in thepresence of an oxidant and at least one non-nucleophilic anion at verylow temperatures. The non-nucleophilic anion is incorporated into thepolymer as a dopant. To prepare such conductive polymers, a solutioncontaining an oxidant/dopant in an organic solvent or aqueous organicsolvent is prepared; the solution is cooled, preferably by means of adry ice bath, to a temperature sufficiently low such that the monomerfor preparing the polymer capable of absorbing microwave energy will notpolymerize prior to its being coated on a substrate; then the monomerfor preparing the polymer capable of absorbing microwave energy isintroduced to the cooled solution; next the resulting solution is coatedonto the substrate. The temperature of the coating is then allowed toreach ambient temperature, whereupon a polymeric coating is formed. Thecoating is then washed with water to remove byproducts and allowed todry.

The temperature of the coating solution must be sufficiently low suchthat the monomer for preparing the polymer capable of absorbingmicrowave energy will not polymerize prior to being deposited on thesubstrate. Polymerization on the substrate can be carried out attemperatures of from about -20° C. to 70° C. Ambient temperatures areconvenient because further heating or cooling will not be required. Atelevated temperatures, the polymerization reaction occurs very rapidly,e.g., in about 10 seconds. Polymerization at lower temperatures, e.g.,from about -20° C. to about 30° C., is preferred for very conductivecoatings. In the case of the monomer for preparing polypyrrole, thecoating solution should be kept below -25° C., preferably below -40° C.Pyrrole, in the presence of an oxidant, begins to polymerize slowly at-40° C. and polymerizes more rapidly as the temperature is increased.

Monomers suitable for preparing polypyrrole include, but are not limitedto, pyrrole, 3-substituted pyrrole, 3,4-disubstituted pyrrole,N-substituted pyrrole, and mixtures thereof, wherein said substituentsare chosen from alkyl groups or aryl groups. Alkyl groups can be linearor branched moieties having up to 12 carbon atoms, and optionallycontaining up to two heteroatoms selected from the group consisting ofoxygen, nitrogen and sulfur.

Organic solvents suitable for the coating solution include aliphaticalcohols having up to and including six carbon atoms, e.g., methanol,ethanol. Organic solvents may contain up to 30% by volume water. Otherorganic solvents useful for the coating solution include 1,3-dioxolane,tetrahydrofuran, and diethyl ether.

Oxidants/dopants suitable for the coating solution include a salt of acationic oxidant, such as, for example (C₆ H₅)₃ C⁺, Fe⁺³, Cu⁺², Ce⁺⁴, incombination with a non-nucleophilic anion, e.g., a salt of a stronginorganic acid, such as FeCl₃, or Fe(ClO₄)₃, a salt of an organic monoor di-sulfonic acid, such as p-toluene-sulfonic acid. Alternatively, anorganic sulfonic acid can be added to the solution in addition to themonomer and oxidant. Other oxidants that can be used in this processinclude the peroxy acids and their salts. The concentration ofoxidant/dopant can vary, with the upper limit determined by thesolubility of the oxidant/dopant at the selected temperature. Theoxidant/dopant can comprise a mixture of an oxidant and dopant or asingle material that functions as an oxidant and dopant.

Although the concentration of the oxidant and the dopant in the solventcan vary, the preferred range of concentrations of the oxidant/dopant insolvent is 2% to 40% by weight.

The concentration of the monomer is determined by the oxidativeequivalent of the oxidant, the preferred ratio of oxidant to monomerbeing approximately 2.2:1 on a molar basis. The duration ofpolymerization prior to the rinse can range from 20 seconds to fourminutes.

These polymers may be applied to the substrate by appropriate coatingtechniques such as gravure coating, roll coating, slot die coating,pattern coating, nozzles, spray applicators, knife coating, and anyother coating means.

The microwave absorbing susceptor layer and substrate may be provided asseparate layers that are placed in intimate physical contact with eachother, but are not fused. In this configuration, the microwave absorbingsusceptor layer would require a separate heat meltable or deformablecarrier layer to provide structural support for the susceptor layersufficient to assemble the article.

A construction comprising a carrier layer is less preferred because itrequires the inclusion of an additional component. Structural supportfor the susceptor layer is preferably provided by the substrate itself,and preferably the susceptor layer is fused to the substrate, forexample, by heat, pressure coating or other appropriate means. Also, thesusceptor layer may be disposed substantially coextensive with thesubstrate or disposed in a predetermined pattern of localized areas.

The present invention also helps to provide more uniform heating of anadherend having a heat sink, i.e., adherends that absorb thermal energyin a non-uniform manner. A heat sink may cause certain regions of theadherend to heat more slowly because thermal energy is transferred tothe heat sink. In contrast, regions not associated with a heat sink willheat rapidly. If the article of the present invention is employed, thoseregions not associated with a heat sink will reach the melt ordeformation temperature of the substrate more quickly and heating willbe slowed in this region. The regions associated with the heat sinkwill, however, continue to heat rapidly until the melt or deformationtemperature of the substrate is reached.

Hot-melt adhesives which are heated using microwave radiation haveseveral advantages over traditionally heated adhesives. An adhesive canbe produced that can achieve its melt-flow temperature in a relativelyshort period of time while at the same time heating up in a very uniformfashion. The hot-melt adhesive may be used in any application where itis desirable to decrease the amount of time required to adhere two ormore adherends together. Preferably, the hot-melt adhesive substratewill achieve its melt-flow temperature or activation temperature in lessthan 50 seconds of exposure to microwave as generated in a 750 wattmicrowave oven.

The melting point of the hot melt adhesive is preferably comparably low,for example, less than 100° C., so that the adhesive may be heatedwithout vaporizing any water available in the adherend.

Surprisingly, the location of the susceptor layer on one side of theadhesive does not interfere with adhesive bonding of an adherend on themicrowave radiation absorbing side of the article. After heating to themelting or activation temperature, the absorbing layer breaks up andallows adhesive to contact both adherends.

An alternate embodiment of the invention provides for a susceptor layerof a predetermined pattern of localized areas on the hot-melt orheat-curable adhesive. Such a limited coating of the microwave absorbingmaterial allows for localized adhesion of parts at a particular locationand no adhesion at other locations, even though the other location maybe in contact with hot-melt or heat-curable adhesive.

The preferred items to be constructed using the microwaveable adhesivesubstrate of the instant invention are buffing pads commonly used in thehigh speed polishing of automobiles and the like. Such buffing pads aredisclosed in U.S. Pat. Nos. 4,607,412, 4,907,313 and 5,001,804, all ofwhich are herein incorporated by reference. This is an excellent exampleof an application where the instant invention is particularly useful,because the pads act as thermal insulators and the central hub acts as aheat sink. Both of those factors create difficulties in the use oftraditional hot-melt or heat-curable adhesives for adhering the separateparts together.

FIG. 2 shows an exploded view of preferred buff 30. Pads 32 and 34comprise plurality of yarn tufts protruding from one side and stitchesexposed on the second side. Pads 32 and 34 are adhered to plastic hub 36by adhesive articles 38, 40, 42 and 44, which represent the articledisclosed wherein the substrate is a hot-melt or heat-curable adhesive;and adhesive 39 and 41, which are hot-melt adhesive films providedwithout a microwave radiation absorbing susceptor layer. Adhesivearticle 38 is preferably provided with the susceptor layer facing intoward the adhesive 39, and adhesive article 42 is preferably providedwith the susceptor layer facing out toward adhesive 39. In this way,adhesive 39 is also melted by the microwave susceptor layer present inadhesive articles 38 and 42. Similarly, adhesive article 40 ispreferably provided with the susceptor layer facing in toward adhesive41, and adhesive article 44 is preferably provided with the susceptorlayer facing out toward adhesive 41. The adhesives and adhesive articlesare preferably generally coextensive with the pad, and preferably atleast 5 cm in diameter. More preferably the adhesives and adhesivearticles are at least 10 cm in diameter, and particularly about 20 cm indiameter.

FIG. 3 shows the construction of the pad shown in FIG. 2, using assembly50. Pads 52 and 54 are adhered to central hub 64 by adhesive articles56, 58, 60 and 62 and adhesives 61 and 63. Microwave transparent disks66 and 68 are placed on the outside of pads 52 and 54. Examples ofmicrowave transparent materials for use in disks 66 and 68 are Teflon™polytetrafluoroethylene and Lexan™ polycarbonate. Each of the abovecomponents is threaded on bolt 70 and are compressed using compressionmeans 72 and 74. For example, compression means 72 and 74 may be nutsthreaded on bolt 70.

In preparing the preferred buff, the stack of components shown in FIG. 3is pressed together and exposed to microwave heating, preferably withpressure. Although pressure during the cooling of the adhesive is notrequired, such continued pressure may provide better penetration of theadhesive into the yarn stitches.

While the central hub may be a solid disk, preferably the hub isprovided with holes to allow adhesive-to-adhesive bonding of theseparate adhesive layers on opposite sides of the hub. The central hubof the buff pad facilitates bonding by acting as a heat sink. Betteradhesive bonds are formed when the temperature of the adherendsapproximates that of the adhesive. Because the central hub acts toabsorb some of the heat energy, the adhesive heats more slowly and theadherends are allowed more time to thermally equilibrate with theadhesive.

FIG. 4 shows graphs of the time versus the temperature of the hot-meltadhesive at the edge (Curve C) and at the center (Curve D) of a buffassembly as it is exposed to microwave radiation.

This figure shows that the initial heating rate is quite large asreflected by the large initial slopes a and e of the curves C and D.Upon reaching the melting transition temperature of the substrate, thecurves show relatively flat regions b and f, then the melted articlecontinues to heat at a much slower rate after the transition temperatureas shown by the attenuated slopes c and g of the curves C and D. Thecenter of the buff pad heats more slowly than the edge because of theheat sink effect of the solid central hub. Thus, a delay in heating atthe center is seen (Region d). The wariness of curves C and D is due tothe oven being used at 47% power. This means that the microwave oven isonly on for 47% of the time and some cooling occurs when it is off. Useof this type of heating cycle allows for heat to be transferred moreuniformly by conduction throughout the object to be heated.

The following examples are provided for illustration purposes only, andare not intended to be limiting as to the scope of the present inventionin any way.

EXAMPLES Example 1

SAMPLE DESCRIPTION: All of the samples in this example were aluminumvapor coated onto 89 micrometer thick Dow DAF 916 hot-melt adhesivefilms.

EXPERIMENTAL METHOD: The measurements were made in a 700 watt LittonGeneration II microwave oven. The samples were cut to 28.6 mm×28.6 mmsquares. The samples were placed on a glass slide. This was placed on abed of glass wool in the center of the microwave oven. More glass woolwas placed over the sample. The purpose of the glass wool was to reducethe sample heat loss. A Luxtron™ MEL fiber optic temperature probe wasplaced in the center of the sample. The time in seconds required for thesample to reach 120° C. was measured.

                  TABLE I                                                         ______________________________________                                        COATING THICKNESS                                                             ANGSTROMS         TIME (SECONDS)                                              ______________________________________                                         0                244                                                          45               244                                                          55               244                                                          77               12.7                                                         90               11                                                          115               10.5                                                        147               8.5                                                         165               14                                                          190               31                                                          230               75                                                          290               93                                                          690               118                                                         2048              109                                                         ______________________________________                                    

FIG. 5 is a graphical representation of the information presented inTable I above. Line A indicates that there is an optimum range of vaporcoat thickness for aluminum coatings for short microwave heating. FIG. 6is an expanded view of the information presented in FIG. 5, wherein lineB shows the heating time required for various vapor coat thicknesses ofaluminum.

Example 2

A conventional lab coater was equipped to handle low temperature coatingsolutions. The coater was additionally equipped with a 20 cm wide singleslot slide coating bar and a corona surface treater. A roll of poly(ethylene-co-acrylic acid) hot-melt adhesive film (DAF 916, availablefrom Dow Chemical Corporation, 30.5 cm wide, 88 micron thick) wasmounted on the coater. A methanol solution containing 25% by weightferric tosylate, precooled to -78° C., was pumped through a static mixerinto the coating bar at a rate of 32 ml/min. Simultaneously, a methanolsolution of 20% by volume pyrrole, maintained at ambient temperature,was injected into the stream, at a point just prior to the static mixer,at a flow rate of 8 ml/min. The flowlines, static mixer, and coating barwere continuously cooled to -70° C. by means of a circulatingfluorocarbon coolant. The mixed solution was coated onto the filmsurface at a web speed of 3.05 m/min. Just prior to passing the coatingbar, the film surface was corona-treated at an energy level of 1.0Joules/cm². As it moved with the web, the wet coating gradually attainedambient temperature, the solvent evaporated, and a coating ofpolypyrrole was formed under a crust of solidified byproducts. Thisprocess was completed within 30 sec. The solidified crust was removed bypassing the film surface over a cylindrical brush, continuously wettedwith a spray of water, and rotated counter to the web direction. Thisrinsing process revealed a uniform, susceptor, transparent coating ofpolypyrrole tosylate. The surface was air dried and the coated filmrewound. In this continuous fashion, 100 m of film was coated. Thesurface resistivity of the coated film was measured to be 2500 ohms/squsing a DELCOM Model 707R non-contact conductance monitor.

Upon application of microwave energy as in Example 1, this film reached120° C. in 5 seconds, and showed no arcing.

EVALUATION OF VARIOUS VAPOR COATING LEVELS Examples 3-12

Samples of Dow DAF 916 adhesive, 3.5 mils thick, were aluminum vaporcoated one side at coating levels of 17, 62 or 3400 ohms/square.

A buff was constructed by stacking in order:

a pad, stitch exposed side up

an adhesive article, vapor-coated side up

an adhesive (3.5 mil Dow DAF 916 adhesive)

an adhesive article, vapor-coated side down

a hub

an adhesive article, vapor-coated side up

an adhesive (3.5 mil Dow DAF 916 adhesive)

an adhesive article, vapor-coated side down

a pad, stitch exposed side down

This buff was placed in a 650 watt Tappan microwave, 2450 MHz, withrotating table. A clamping fixture consisting of 10.2 cm air cylinderwith two polycarbonate plates for clamping, capable of applying1112-5026 newtons on buff, was used to compress the buff during heating.

Buffs are assembled, each with a different level of vapor coating. Theexperiment is a 3 factorial experiment with vapor coating, microwavingtime and clamping as the three variables. There are three responses thatare tested: 1) % tufts bonded (sample of 10 tufts per buff areexamined); 2) % of hub not melted (visual measurement); and 3) % bond ofadhesive to bond (visual measurement).

    ______________________________________                                                 Pressure    Time     Vapor Coating                                   Example  (kPa)       (min-sec)                                                                              (ohms/square)                                   ______________________________________                                        3        137.9       1 15     3400                                            4        620.5       1 15     3400                                            5        137.9       1 15     3400                                            6        620.5       2 30     3400                                            7        137.9       2 30      17                                             8        620.5       1 15      17                                             9        137.9       1 15      17                                             10       620.5       2 30      17                                             11       379.2       1 53      62                                             12       379.2       1 53      62                                             ______________________________________                                    

    ______________________________________                                        RESULTS                                                                       Example % Tuft Bonds                                                                             % Hub Not Melted                                                                            % Bond to Hub                                ______________________________________                                        3       0          100            0                                           4       10         100            5                                           5       90          75            98                                          6       80          98           100                                          7       0          100            0                                           8       40         100            1                                           10      10          95           100                                          11      90         100           100                                          12      80         100           100                                          ______________________________________                                    

Results shown an air cylinder pressure of 379.2 kPa or greater isacceptable. Although all vapor coating levels did work to melt theadhesive, the 62 ohms per square is preferred for buff construction.

Examples 13-14 Influence of Vapor Coating on Heating Time

Comparative Example 13 is 3.5 mils thick Dow DAF 916 film that has notbeen vapor coated.

Example 14 is same film adhesive but aluminum vapor coated to 62ohms/square.

Equipment

650 watt Tappan microwave, 2450 MHz, with rotating table.

The examples were placed in the oven. The microwave was turned on atfull power and left on until samples were melted.

    ______________________________________                                        Example 13     Example 14                                                     Standard Adhesive                                                                            Vapor Coated Adhesive                                          ______________________________________                                        10 minutes     5 seconds                                                      ______________________________________                                    

Examples 15-17

Buffs are assembled as described in Example 11. Three types of heatingmethods were used:

15. The buff is clamped in a fixture and the assembly is then put in aconvection oven set at 121° C. A thermometer is inserted into the buff.Then record the time it takes for the adhesive to reach melt point (110°C.).

16. The buff is heated in a heated press. The two press platens areheated to 162° C. A thermometer is inserted into the buff. Compress buffwith 181.4 kg pressure. Recorded the time it takes for the adhesive toreach melt point (110° C.).

17. The buff is clamped in a fixture and put into 650 watt microwave asdescribed in Examples 3-12.

    ______________________________________                                        Example Heating Method Time to Reach Melt Point                               ______________________________________                                        15      Convection Oven                                                                               3 hours                                               16      Heated or Hot Platten                                                                        20 minutes                                                     Press                                                                 17      Microwave       1 minute 35 seconds                                   ______________________________________                                    

Example 18

Dow DAF 916 hot-melt adhesive was vapor coated with aluminum to athickness of about 250 angstroms using a mask to create patterns ofvapor coated areas. The mask consisted of paper with several squares6.35 mm, 12.7 mm and 25.4 mm per side cut in the paper.

Some vapor coated squares were placed in a 650 watt kitchen grademicrowave and heated for a few seconds. The 25.4 mm and 12.7 mm squaresmelted, but the 6.35 mm squares did not. Even when heated for oneminute, the 6.35 mm squares showed only marginal signs of melting. clExample 19

The experiment of example 18 was repeated in a 6 kW oven manufactured byRaytheon, wherein a dummy water load was placed in the oven forprotection. The water load absorbed approximately 3 kW of energy,leaving 3 kW available for absorption by the adhesive article.

The 25.4 and 12.7 mm squares began melting with the oven on for only 1second. The 6.35 mm squares did not melt even when the oven was on for30 seconds. Strips were cut 6.35 mm×12.7 mm, 6.35 mm×25.4 mm, 6.35mm×50.8 mm and a very narrow strip approximately 0.8 mm×50.8 mm. All butthe very narrow strip melted very quickly. In each case where meltingoccurred, there was no melting of the adhesive immediately adjacent tovapor coated areas.

For this particular adhesive and vapor-coating thickness, the coatedarea is preferably greater than 0.40 cm².

Experiment 20

FIG. 7 shows the heating curves of various compositions of hot-meltadhesive when subjected to microwave radiation.

Curves h and i depict the heating rate of hot-melt adhesive that hasbeen vapor coated with aluminum, one embodiment of the presentinvention.

Curve j shows the heating rate of aluminum vapor coated hot-meltadhesive that has been melted prior to microwave irradiation by beingplace in a convection oven. The preheated vapor coated adhesive wasallowed to cool before being subjected to microwave radiation.

Curve k shows the heating rate of non-vapor coated hot-melt adhesive.Like the sample of curve j, this sample was heated until melted in aconvection oven and cooled before the sample was subjected to microwaveradiation.

The curves show that unmelted vapor coated hot-melt adhesive heats morerapidly than either non-vapor coated hot-melt adhesive or vapor coatedhot-melt adhesive that has been previously melted.

Both the premelted vapor coated adhesive and the premelted non-vaporcoated adhesive show little microwave absorption and appear relativelytransparent, demonstrating that the disclosed article heats rapidlyuntil the melting or deformation temperature is reached and becomesrelatively transparent to microwave radiation thereafter.

EXPERIMENTAL

All adhesive samples were Dow DAF 916 adhesive. The premelted vaporcoated hot-melt adhesive was obtained by melting eight sheets of 3.5 milthick adhesive of about 20 cm in diameter (approximately 20 g) that hadbeen vapor coated with about 140 angstroms of aluminum. The sheets weremelted in a standard laboratory oven at about 177° C., with occasionalstirring. Similarly, an equivalent amount on non-vapor coated adhesivewas melted.

A small hole was bored into each melted sample and a Luxtron™ MEL fiberoptic temperature probe was inserted into each hole. In addition, eightsheets of vapor coated adhesive were stacked and folded together. Twoprobes were inserted into this sample.

All three samples were placed in a Raytheon industrial microwave oven(6.4 kW, 2450 Mhz) and were simultaneously exposed to microwaveradiation. The oven was operated at 47% power level (Cycle time of 47%on and 53% off). The temperatures of the samples were monitored with theinserted temperature probes and the microwave was stopped when thehighest temperature went above 300° C.

We claim:
 1. A microwave actuable self-limiting adhesive article for usein a microwave oven which can be used to bond adherends, comprising:a) asubstrate consisting of a hot-melt or heat-curable adhesive; and b) amicrowave susceptor layer of at least electrically semi-conductivemicrowave radiation absorbing material that is disposed on at least aportion of said substrate such that said substrate provides physicalsupport to said susceptor layer, said susceptor layer being responsiveto exposure to microwave radiation for raising the temperature of saidsubstrate above a desired level sufficient to melt said substrate andthereby deactivate said susceptor layer by destruction of said physicalsupport through melting of said substrate when said desired temperaturelevel is achieved.
 2. The article of claim 1, wherein said substratemelts at a temperature between 60° and 150° C.
 3. The article of claim1, wherein said substrate melts at a temperature between 80° and 120° C.4. The article of claim 1, wherein the substrate is selected from thegroup consisting of ethylene vinyl acetate, ethylene ethyl acrylate,ethylene acrylic acid, ethylene methacrylic acid, polyethylene,polyamides, ethylene vinyl esters and mixtures thereof.
 5. The articleof claim 1, wherein the substrate is selected from the group consistingof alkyd, coumarone-indene, phenolic, rosin and terpene resins andmixtures thereof.
 6. The article of claim 1, wherein the substrate isselected from the group consisting of heat activatable phenolic resins,amino resins, nitrile rubbers, neoprene rubbers, epoxy resins,isocyanate polymers, vinyl monomers and mixtures thereof.
 7. The articleof claim 1, wherein the substrate is an ethylene acrylicacid/polyethylene blend.
 8. The article of claim 1 wherein saidsusceptor layer has a sheet resistivity of between 5 and 5000 ohms persquare.
 9. The article of claim 1 wherein said susceptor layer has asheet resistivity of between 15 and 1000 ohms per square.
 10. Thearticle of claim 1 wherein said susceptor layer comprises less than 1%by weight of said article.
 11. The article of claim 1 wherein saidsusceptor layer comprises less than 0.01% by weight of said article. 12.The article of claim 1 wherein the susceptor layer has a sheetresistivity greater than 0.2 ohm per square.
 13. The article of claim 1wherein said susceptor layer is a metal selected from the groupconsisting of iron, nickel, stainless steel, tin, titanium, magnesium,aluminum, silver, tungsten, copper, gold and chromium.
 14. The articleof claim 1 wherein said susceptor layer is between about 60 and 1000angstroms thick.
 15. The article of claim 1 wherein said susceptor layeris between about 70 and 170 angstroms thick.
 16. The article of claim 1wherein said susceptor layer is a polymeric coating selected from thegroup consisting of polypyrroles, polyanilines and polythiophenes. 17.The article of claim 1 wherein the susceptor layer is substantiallycoextensive with the substrate.